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Riddle me this: When is a SAS drive not a SAS drive?

I don't just want SAS drives, I also want SATA drives. SATA drives are cheaper than SAS drives.

Nearline SAS drives are just SATA drives with some sort of converter on them.

So is this right? Is this the actual situation?

First up, if your storage uses a SAS based controller with a SAS backplane, then normally you can plug SAS drives into that enclosure, or you can plug SATA drives into that enclosure. This is great because when you plug SATA drives into a SAS backplane, you can actually send SCSI commands to the drive plus you can send native SATA commands t00 (which is handy when you are writing software for RAID array drivers).

But (and this is a big but) what we do know is that equivalent (size and RPM) SAS drives perform better than SATA drives. This is because:

SAS is full-duplex, SATA is simplex.

SAS uses the native SCSI command set which has more functionality (which leads to the next point).

A SAS drive uses SCSI error checking and reporting which is much more robust than the SATA error reporting. This allows your storage system to collect richer information from the drive if errors are occurring (such as a failing or marginal disk).

SAS drives are dual ported which is vital in dual controller enclosures.

So given a choice (and a very small price differential), why choose SATA over SAS? SAS is the clear winner. What we should instead differentiate on is speed (7.2K RPM vs 10K RPM vs 15K RPM vs SSD) and size (2.5" vs 3.5" form factor).

Which leads us to Nearline SAS

It is a common belief, that if you buy a Nearline SAS (or NL-SAS) drive it is really a SATA drive with a SAS connector (interposer) stuck on it. But this is confusion from the past.

What led to the confusion?

Most midrange and enterprise storage controllers and enclosures up until recent years, used disks that had fibre channel interfaces on them. We plugged those disks into fibre channel enclosures. Examples include the DS4700 or the DS8100. And yet these devices also offered SATA drives. How did they do this?

They took a SATA drive and added a SATA to Fibre Channel converter card to the disk. We call this extra piece of hardware an interposer or bridge card. So people start assuming that this is common practice in every product. In fact we are now seeing SAS drives being put into fibre channel disk enclosures by using a SAS to fibre channel interposer.

There are in indeed older products that did take a SATA drive and add a SATA to SAS interposer to achieve a similar thing. But that really is not necessary any more. The reason? The same hard drive can now be ordered from the factory as either a SAS drive or a SATA drive.

Seagate have a nice selector tool to let you see all their possible combinations. For instance you can order a 2 TB drive with a 6 Gbps SAS interface, which is a model ST32000444SS:

Or you can order a 2 TB drive with a 6 Gbps SATA interface, which is a model ST2000NM0011:

So what you get is very similar drive hardware (same spindles, heads, motors) but with different adapter hardware, built with the desired adapter at manufacture time. Meaning that if we install this drive into a SAS enclosure, there is no need to add an interposer or bridge card to the drive after you bought it.

This leads to the next question:

OK. So this is good, so Nearline SAS drives are MADE as SAS drives. Does that mean a drive manufactured with a SAS adapter is a SAS drive or a Nearline SAS drive?

Now we are mixing up two different things. SAS as a standard is a combination of a connection technology (theSerialAttached part) and a command set (the SCSI part). Actually SCSI as a standardalso defines both connection methods and command sets. So SAS is really talking about how we connect to the disk and what command set we use to control the disk.

Nearline on the other hand is a statement about the disks rotational speed and it's mean time between failure (MBTF). A Nearline-SAS drive isNearline because:

It rotates slower (7200 RPM) than the higher specified Enterprise drives (that spin at 10 K or 15K RPM). Because they are slower they can also hold way more data.

It has a lower MBTF (1.2 million hours) than the higher specified Enterprise drives (which are normally specified at 1.6 million hours).

So we have now gone full circle. A Nearline-SAS drive can use the same physical disk hardware as a SATA drive, but with a superior adapter that uses a superior command set, built onto the drive at manufacture time.